ZX Spectrum Issue 6a short

[Rabs]

Hi, ZX spectrum issue 6a short.

Initially this Issue 6a indicated no -5 or +12v. Found TR4 to be faulty.

TR4 and TR5 changed (although TR5 was ok).

Initially Retroleum X-ROM board indicated +5, -5 and +12v for a maybe a minute but then suddenly my bench power supply started to current limit.

In circuit resistance checks for lower RAM inidicate Vcc 900 ohms, Vdd 2k and Vbb 5k so can't see a short there.

In circuit diode test on D11, D12 and D19 passes. Removed and checked D15 and D17 and passed.

Retroleum X-ROM board now very briefly indicates +5, -5 and 12v before current limit kicks in at 800mA.

I am thinking capacitors but any help much appreciated

Thanks
Rob

[Rabs]

Replaced the two 100uF caps C44, C45 and now have a stable voltage and current.

Which fo you reckon was the problem or both?

[Rabs]

Well that did not go well having checked the voltages +5 was more like +7 and +12 was more like +17 . No idea what damage has been done yet.

Checked the 7805 and it was kicking out 7V. Should have checked this in the first place. Rule 1 check input voltage.

Anyway replaced with a TSR 1-2450 and I am now back at square 1 a short somewhere and no -5 or +12. So back to the start I go.

[Rabs]

ULA was the short or at least without this I get a stable +5 and +12, -5 is a little low at -3.8V.

[1024MAK]

The ULA only runs off the +5V. And indeed, the actual supply to the core goes via resistor R34 to pin 13. The ULA has no direct connections to the -5V or +12V supplies. ULA pin 14 just supplies the peripheral interface circuitry.

The normal cause of problems with the -5V or +12V supplies, TR4 cooking itself, or a bench power supply hitting a current limit, is one or more 4116 DRAM chips breaking down when powered up. The short may not reveal itself when just doing resistance tests.

If you are lucky, it will be whichever 4116 DRAM chip is running the hottest.

Capacitors that I consider critical
I recommend renewing these:

Issue 4A, 4B, 4S, 5 and 6A boards
C34 22uF 10V or 16V
C44 100uF 16V
C45 100uF 16V
C46 1uF 50V or 63V
C50 22uF 16V
C74 4.7uF 10V or 16V
C78 22uF 25V (where fitted)
C79 1uF 50V or 63V
C80 22uF 25V (where fitted)

Mark

[Rabs]

Thanks Mark,

My original diagnosis was TR4 and on testing this was faulty. But I should have also checked the 7805 as this was kicking out 7V. So that has been changed also with a TSR 1-2450.

Initially this looked OK but the -5V was low, so I started a capacitor replacement.

After replacing C79 and C80 my bench power supply voltages dropped. Something is wrong.

Thinking it is more likely I am still chasing a short on one of the 4116 DRAM chips but no indications which, at least none are warm.

So guess I will remove all the 4116 DRAMs and check voltages again.

Picture just in case you spot anything.

Thanks
Rob

[Rabs]

So, I was lucky. IC13 removed still same, IC12 removed and now I have stable voltages and all correct.

Will Plug ULA back in a see what happens.

[Rabs]

OK voltage and current ok with the ULA back in but diagnostic ROM is not running.

Time to get the scope out and see what is alive.

[Rabs]

Well not much is running X1 is dead but X2 is alive a least. This could be a long process.

[Rabs]

Getting somewhere now swapped ULA 6C001E-7 with an earlier 5C112E-3 and it's up and running the Diagnostics but tv out signal is a bit rubbish.